1. Field of the Invention
The present invention relates to a structure of a electric rotating machine and a driving method thereof, and particularly relates to an inexpensive electric rotating machine which is the most suitable for a scanner motor of a copying machine or the like, and which is required to be low in vibration as well as in rotational irregularity.
2. Description of the Prior Art
An inner-rotor 3-phase 6-slot permanent-magnet type stepping motor (hereinafter simply referred to as "3-phase electric rotating machine"), which is one of conventional multi-phase permanent-magnet type electric rotating machines, has such a structure, for example, as shown in FIG. 15.
FIG. 15 is a conceptual sectional view schematically illustrating the structure of an inner-rotor 3-phase electric rotating machine, in which the reference numerals 10 and 20 represent a stator and a rotor, respectively. A rotation shaft, a housing and so on are omitted in the drawing.
The rotor 20 is rotatably supported by a not-shown rotation shaft and bearings. In FIG. 15, the reference character N represents an N (north) pole of a permanent magnet which is magnetized so that the N-pole faces the stator, and S represents an S (south) pole of a permanent magnet which is magnetized so that the S-pole faces the stator. Thus, two pairs of magnetic poles are formed.
The stator 10 has main poles 10a1, 10a2, 10a3, 10a4, 10a5 and 10a6 formed at a predetermined distance from the surface of the rotor 20, and exciting coils 10b1, 10b2, 10b3, 10b4, 10b5 and 10b6 are wound in the same direction on the main poles 10a1, 10a2, 10a3, 10a4, 10a5 and 10a6, respectively.
FIG. 16 shows an example of the connection of the above-mentioned coils 10b1, 10b2, 10b3, 10b4, 10b5 and 10b6. The reference numerals 10b1, 10b2, 10b3, 10b4, 10b5 and 10b6 shown in FIG. 16 are corresponding to the reference numerals of the coils shown in FIG. 15 respectively.
Specifically, for example, let the winding start of the lead wire of the first coil 10b1 be a U-terminal of this electric rotating machine, then the winding end of the first coil 10b1 is connected to the winding start of the fourth coil 10b4. Let the winding start of the lead wire of the second coil 10b2 be a V-terminal of the electric rotating machine, then the winding end of the second coil 10b2 is connected to the winding start of the fifth coil 10b5, and let the winding start of the lead wire of the third coil 10b3 be a W-terminal of the electric rotating machine, the winding end of the third coil 10b3 is connected to the winding start of the sixth coil 10b6.
The winding ends of the other coils 10b4, 10b5 and 10b6 are connected to each other.
That is, the exciting coils of this electric rotating machine form a star connection.
FIG. 17 shows an exciting method of the electric rotating machine having such a structure as shown in FIGS. 15 and 16.
The numerical values 1 to 6 in the leftmost column in FIG. 17 show the order of exciting steps from top to bottom, returning to the step 1 after reaching the step 6.
The characters U, V an d W in the uppermost row show the terminals shown in FIG. 16.
The symbol (+) shows a predetermined direction of an electric current, that is, the direction of the current flowing into the above-mentioned terminals, and (-) shows the opposite direction, that is, the direction of the current flowing out of the terminals.
Specifically, the step 1 shows that a current of a predetermined value is made to flow from the terminal U to the terminal V. Therefore, in the step 1, for example, the first main pole 10a1 and the fourth main pole 10a4 become N-poles, and the second main pole 10a2 and the fifth main pole 10a5 become S-poles.
Similarly to this, the step 2 shows that the current of the same value is switched so as to flow from the terminal W to the terminal V so that the third main pole 10a3 and the sixth main pole 10a6 become N-poles and the second main pole 10a2 and the fifth main pole 10a5 become S-poles. By the change of the terminals to which the exciting current is supplied as shown in FIG. 17, the electric rotating machine rotates at a rotational speed in accordance with the stepping speed of the applied steps with the stepping angle of 30 degrees which is 1/6 of the pitch angle of 180 degrees because the number of pairs of the main poles is 2.
In a driving means for supplying such a constant current rectangular wave as shown in FIG. 17, such a torque as shown in FIG. 18 is generated in this electric rotating machine. The torque is expressed by the following expression (3) is established and the input power can be expressed by the expression (4).
In FIG. 18 and the expressions (3) and (4), T.sub.1 represents one phase of torque generated in a main pole where a total current flows; T.sub.2, a total torque in the case of two-phase excitation; P.sub.12, input power; R, a resistance component of a coil; and I, the value of the exciting current. ##EQU1## Therefore, when efficiency T.sub.2 /P.sub.12 is represented by K.sub.2, the following expression (5) is established. ##EQU2## K.sub.2 shows the efficiency in the case of the two-phase excitation.
It has been inevitable that the above-mentioned electric rotating machine is disadvantageous in the economical viewpoint in that the coils should be wound on all the main poles though there is generated no unbalanced electromagnetic force, because the number of coils is six while the number of input terminals is three, as shown in FIGS. 15 and 16.
Further, FIG. 19A shows another example of connection in which the number of coils is three. In this case, the three coils are wound on three main poles respectively, and there arises such a problem that an unbalanced electromagnetic force is generated to cause vibrations, as will be described later with reference to FIG. 19B.
Specifically, three coils 100b1, 100b2 and 100b3 are wound on main poles 100a1, 100a3 and 100a5 respectively, with their respective terminals U, V and W as shown in FIG. 19A. Then, when a current is supplied from the terminal U to the terminals V and W, the current flows in the coils 100b1, 100b2 and 100b3 in the directions as shown by arrows, and the main pole 100a1 becomes an N-pole while the main poles 100a3 and 100a5 become S-poles.
In FIG. 19B, therefore, if the radial force acting on a rotor by the main pole 100a1 is represented by F.sub.a1, the radial force acting on the rotor by the main pole 100a3 is represented by F.sub.a3, and the radial force acting on the rotor by the main pole 100a5 is represented by F.sub.a5, the resultant force F.sub.T is generated by the radial force F.sub.a3 acting on the rotor by the main pole 100a3 and the radial force F.sub.a5 acting on the rotor by the main pole 100a5. It is apparent that the resultant force F.sub.T is smaller than the radial force F.sub.a1. Therefore, the rotor receives a radial force in the direction of the main pole 100a1 by the force which is a difference between F.sub.a1 and F.sub.T. This radial force rotates in accordance with the exciting steps to make the rotor vibrates.
In addition, an exciting means which is superior in efficiency to that shown in the expression (5) has been required.
It is therefore an object of the present invention to solve the foregoing problems so as to provide a multi-phase permanent-magnet type electric rotating machine in which the number of coils is reduced by half without generating any vibrations while the machine has necessary characteristics corresponding to those of conventional machine, and to provide an exciting means superior in efficiency.